Servo-mechanisms for controlling the position of vehicle control surfaces are not novel. However, the prior art has come to recognize a number of problems in the available systems.
For purposes of discussing the problems, one can consider the rudder of a vessel as a controlled surface. In some senses, of course, the rudder of a vessel is also a vessel control surface since it controls the heading of the vessel. In this application the word control surface or controlled surface may be used interchangeably depending upon the context. The vehicle operator can, by manipulating the helm, call for changes in the rudder position. Conventionally, the servo-system includes an electronic portion which compares an electrical signal representative of the position of the helm with another electrical signal representative of the actual rudder position and develops an error signal to control the rudder position. Furthermore, the error signal may then be used to control a valve in a hydraulic system which actually moves the rudder in response to the developed error signal. It is not uncommon, in these systems, for the position error signal to be so large as to open completely the valve so that other elements of the hydraulic system actually limit the rate of movement of the rudder. Since this hydraulic system conventionally supplies hydraulic power for a variety of other vehicle systems the condition of the hydraulic system is not constant. As a result, the rate of rudder movement may vary in accordance with the condition of the hydraulic system. This is clearly an undesirable condition and the specifications of many vehicle control systems include the specification of rate of movement of the controlled member. Clearly, prior art systems such as that outlined above are inadequate to meet the specification of controlled member velocity.
Furthermore the signals representative of helm and control surface position are generated by transducers which convert mechanical position to electrical signals. Non-linearities in the transducer characteristics will obviously affect operation of the system. Although matched non-linearities in transducers will negate each other under static conditions, such non-linearities in dynamic operation will also affect the control surface velocity. Thus, to meet control surface velocity specifications it may become necessary to provide apparatus to compensate for the dynamic non-linear effects.
In particular synchros are common servo-system transducers. Since synchro signal generation is proportional to the change in sine the synchros non-linearities occur at large (&gt;30.degree.) angular displacement.
Additionally it is desirable to provide failure sensitive devices in the servo-system. This is especially important in large vehicles such as large ships or planes. Clearly, the size of these vehicles multiplies the consequences of accidents. Furthermore although an operator could be relied on to detect failures, the inertia and necessary actuating forces associated with control surfaces magnify the effects of delay in sensing control system failures. For this reason electronic devices are indicated to monitor system operation and take appropriate action in case of system malfunction.